Geology of Cook Islands

Holiday makers, tourists and scuba divers do not realise that there is much for
the speleologist to search for in the Cook Islands. But that is the case.

The Cook Islands have a total land area 241km², however this land is
scattered over 2 million sq km of sea, an area as large as Western Europe. The
Cooks are south of the equator, just east of the International Date Line. The
15 islands are divided into a northern and a southern group, separated by 1000
km of empty sea. The northern island group are older, low lying, coral atolls.
The northern islands have no significance regarding karstification or caves.

The southern islands are generally larger, constituting about 90% of the total
area. They are the continuation of the Austral Islands, lying along the same
north-east south-west fracture of the earth's crust. The largest island,
Rarotonga (67.2km²), is basically a mountainous, volcanic island up to 650m
asl. Aitutaki Island has one small hill around the atoll reef. It is known as
"almost atoll".

Four of the Southern Group of islands, the so called "raised islands" have a
remarkable topography showing signs of karstification and cave development.
These islands are: Atiu, Mauke, Mitiaro and Mangaia.
The islands are located
about 200-250km from the main island. They were formed about 10 million
years ago in the distant geological past, as volcanic islands and gradually
became encircled by coral reefs. Over time the volcanic cones sank and the
Pacific plate moved on. Two million years ago more volcanic activity occurred.
This time the main island, Rarotonga, appeared. The same activity caused a
buckling of the sea floor in the area nearby, causing the four islands to be
raised above sea level. This exposed the coral reefs, which became fossilized
coral limestone, forming the rugged coastline and the low lying plateaus. In
the last 100,000 years a narrow belt of young coral reefs developed around most
of the islands. In the central part of the islands are depressions in the
middle of the limestone plateaus. Here can be found the remains of the volcanic
rocks, which are covered by fertile soil. These depressions are very important
for the supply of fresh water. Between the depressions and the limestone
plateaus, swampy rims were formed. The islands can be described as being like a
very low brimmed hat with a flat outer rim.

The fossilised coral limestone has undergone intense karstification. The
climate is humid and hot. The yearly rainfall is about 3,000-4,000mm. From
December to March 2,500mm rain is expected and the average temperature at this
time of year is around 29°C. The vegetation on the limestone plateaus is
thick, low growing, tropical scrub jungle. As a result of the tropical
limestone solution and the characteristics of the coral limestone, very rugged
surfaces have developed. This is the so called "makatea". The makatea covering
the plateau surfaces hides a razor sharp 1-3m high mass of limestone rocks.
Between these are deep, vegetation covered fissures. This makes it extremely
difficult to gain access to the caves. The most important piece of equipment is
a thick walking stick, which serves to find the dangerous fissures.

The limstone plateaus resemble a sponge as a consequence of the combined
solutional effects of the infilitrating rain water and the tropical vegetation.
This is the basis for cave development. In the islands several hundred minor
caves and cavities exist. In some favourable cases extensive caves with large
chambers have formed. Due to the frequent changes to the sea level in the last
15,000 years, some of the caves are now beneath the water table.